Cathode device having smaller opening

ABSTRACT

A cathode device includes a plurality of emitter tips having conical tip end portions to emit electrons therefrom. A gate electrode layer has an opening through which the tip end portion of each of the emitter tips is exposed. The diameter of the opening in the gate electrode layer is made smaller than that of the portion of the emitter tip at the juncture thereof with the substrate. In the fabrication of the cathode device, an oxide layer is formed at least on the surface of the formed emitter tip to sharpen the latter. By removing the oxide layer, an inner circumferential wall of the opening of the gate electrode layer is formed on the outside of the conical tip end portion of the emitter tip and extends approximately in parallel to the conical tip end portion of the emitter tip.

This is a division of application Ser. No. 08/554,032 filed Nov. 6,1995, now U.S. Pat. No. 5,576,594, which in turn is a continuation ofapplication Ser. No. 08/259,694, filed Jun. 13, 1994, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cathode device, and in particularly,to a cathode device referred to as a small-sized field emission coldcathode. The present invention also relates to a method for fabricatinga cathode device using etching.

2. Description of the Related Art

A small-sized field emission cold cathode comprises an emitter tip or aplurality of emitter tips and a gate electrode layer. A typical cathodedevice is shown in FIG. 28 of the attached drawings. In FIG. 28, thecathode device comprises an emitter tip 2 formed on a substrate 1 andhaving a conical tip end portion 2A, and a gate electrode layer 4 formedabove the substrate 1 and having an opening 4A through which the tip endportion 2A of the emitter tip 2 is exposed. An insulating layer 3 isprovided between the substrate 1 and the gate electrode layer 4. Whenthe voltage is applied between the stem portion 2B of the emitter tip 2and the gate electrode layer 4, a strong electric field is developedbetween the tip end portion 2A of the emitter tip 2 and the innercircumferential wall 4B of the opening 4A of the gate electrode layer 4.Electrons are thus emitted from the tip end portion 2A of the emittertip 2.

The small-sized field emission cold cathode is a source of emittedelectrons and is used in a display device or in a micro-vacuum-tube. Thesmall-sized field emission cold cathode has high electron mobility ofand can operate at a high speed. Also, the small-sized field emissioncold cathode can operate at a high temperature and has high durabilityagainst radiation. Accordingly, it is expected that the small-sizedfield-emission cold cathode can be used in the above describedapplications and also in a variety fields such as microwave elements,super high speed calculators, active devices for use in a radioactiveenvironment in space or in a reactor, or in active devices for use in ahigh-temperature environment.

It is known that the foregoing cathode device can be fabricated usingetching. For example, a typical process for fabricating the cathodedevice on a silicon substrate 1 is shown in FIGS. 29A to 29D in theattached drawings. A mask 51 having a diameter corresponding to theshape of the emitter tip 2 is formed on the substrate (FIG. 29A), andetching is performed so that the unmasked portion of the surface of thesubstrate 1 is removed while the portion of the substrate 1 under themask 51 is left in the form of a peak (FIG. 29B). The peak-shapedportion becomes the emitter tip 2. Thereafter, the substrate 1 issubjected to a thermal oxidation while the mask 51 is maintained on theemitter tip 2, with the result that an oxide diffuses inside the emittertip 2 and an oxide layer 52 is formed on the surface of the emitter tip2 (FIG. 29C). If the oxide layer 52 is removed at the later step, theemitter tip 2 having the sharper tip end portion 2A appears.

Thereafter, the insulating layer 3 is formed while the mask 51 ismaintained on the emitter tip 2. Since the emitter tip 2 is covered bythe mask 51, the insulating layer 3 is formed on the unmasked portion ofthe substrate 1 and on the mask 51. The gate electrode layer 4 is thenformed on the insulating layer 3 by vapor deposition or the like (FIG.29D). The gate electrode layer 4 is placed on the insulating layer 3 onthe substrate 1 and on the insulating layer 3 on the mask 51. When themask 51 is finally removed, the insulating layer 3 and the gateelectrode layer 4 on the mask 51 are removed simultaneously with themask 51. The oxide layer 52 around the emitter tip 2 is also removed.Thus, openings 4A corresponding to the mask 51 are formed in theinsulating layer 3 and the gate electrode layer 4 and the emitter tip 2is exposed through the openings 4A, as shown in FIG. 28.

In the cathode device fabricated as described above, the size "D" of aportion of the emitter tip 2 at the juncture (bottom) thereof with thesubstrate 1 is smaller than the size of the mask 51. The size "d" of theopening 4A of the gate electrode layer 4 depends on that of the mask 51.Therefore, the size "D" of the juncture portion of the emitter tip 2 issmaller than the size "D" of the opening 4A of the gate electrode layer4 (D<d). In practice, when the insulating layer 3 is formed by vapordeposition, the contour of the insulating layer 3 on the mask 51diverges as it piles up, as shown in FIG. 30. In some cases, aparasitically growing collar 10 is formed on the shoulder of the emittertip 2. Upon the subsequent formation of the gate electrode layer 4, theopening 4A of the gate electrode layer 4 becomes larger in diameter thanthe mask 51. When the mask 51, the insulating layer 3 thereon, and theparasitically growing collar 10 are then removed, the size "D" of thejuncture portion of the emitter tip 2 is considerably smaller than thesize "d" of the opening 4A of the gate electrode layer 4 (D <d), asshown in FIG. 31. In addition, it is difficult to make a mask 51 havinga diameter smaller than, for example, approximately 1 μm using currentphotolithographic technology.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a cathode device and amethod for fabricating a cathode device by which it is possible toinduce the field emission with an applied voltage as low as possible.

Another object of the present invention is to provide a display deviceincluding such a cathode device.

A cathode device according to the present invention comprises asubstrate, at least one emitter tip formed on the substrate and having aconical tip end portion, and a gate electrode layer formed above thesubstrate and having an opening through which the tip end portion of theemitter tip is exposed. The cathode device is characterized in that thediameter of the opening of the gate electrode layer is smaller than thatof a portion of the emitter tip at a juncture thereof with thesubstrate.

In this cathode device, the diameter of the opening of the gateelectrode layer is smaller than that of the juncture portion of theemitter tip with the substrate. The tip end portion of the emitter tipis therefore located close to the inner circumferential wall of theopening of the gate electrode layer. Accordingly, field emission can beinduced at a voltage considerably lower than the value which may benecessary in the prior art.

A display device according to the present invention comprises a cathodedevice having the above described features and an anode receivingelectrons emitted from the cathode device.

A method for fabricating a cathode device, in particular, suitable forfabricating the foregoing cathode device, comprises the steps of formingat least one emitter tip having a conical tip end portion on asubstrate, forming an oxide layer at least on the surface of the formedemitter tip, forming a gate electrode layer such that a material of thegate electrode is deposited at least to the oxide layer on the surfaceof the conical tip end portion of the emitter tip and the gate electrodelayer has an opening through which the tip end portion of the emittertip is exposed, and removing the oxide layer on the surface of theemitter tip so that an inner circumferential wall of the opening of thegate electrode layer is formed on the outside of the conical tip endportion of the emitter tip and extends approximately in parallel to theconical tip end portion of the emitter tip.

In this method, the inner circumferential wall of the opening of thegate electrode layer is formed in a substantially truncated coneextending substantially in parallel to the conical tip end portion ofthe emitter tip around the tip end portion. It is therefore possible toobtain the gate electrode layer having an opening diameter smaller thanthat of the stem portion of the emitter tip. Consequently, it ispossible to fabricate a cathode device capable of triggering fieldemission at a very low voltage.

With the development of the cathode devices, it is demanded that thefield emission is induced at as low a voltage as possible. In the tryingan effort to induce field emission at the lowest possible voltage, theinventors have found that the opening of the gate electrode layerthrough which the emitter tip is exposed should be minimized in diameterso that the inner circumferential wall of the opening in the gateelectrode layer can be located closer to the emitter tip. Also, theinventors have found that the height of the inner circumferential wallof the opening of the gate electrode layer must be as high as possibleand that the area of the inner circumferential wall of the openingsurrounding the tip end portion of the emitter tip must be as large aspossible. The present invention satisfies these requirements.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more apparent from the followingdescription of the preferred embodiments, with reference to theaccompanying drawings, in which:

FIG. 1 is a cross-sectional view of a cathode device according to anembodiment of the present invention;

FIG. 2 is a schematic cross-sectional view illustrating an example ofusing the cathode device of FIG. 1;

FIG. 3 is a schematic perspective view illustrating a display unit inwhich the cathode device of FIG. 1 is used;

FIG. 4 is a cross-sectional view illustrating one step of the firstembodiment of the method for fabricating a cathode device according tothe present invention;

FIG. 5 is a cross-sectional view illustrating the next step after thatin FIG. 4;

FIG. 6 is a cross-sectional view illustrating the next step after thatin FIG. 5;

FIG. 7 is a cross-sectional view illustrating the next step after thatin FIG. 6;

FIG. 8 is a cross-sectional view illustrating the next step after thatin FIG. 7;

FIG. 9 is a cross-sectional view illustrating the next step after thatin FIG. 8;

FIG. 10 is a cross-sectional view illustrating the next step after thatin FIG. 9;

FIG. 11 is a view illustrating the relationships between voltage andcurrent in the cathode devices of the present invention and in the priorart;

FIG. 12 is a view illustrating the field emission characteristic of thecathode devices of the present invention and the prior art;

FIG. 13 is a view illustrating the relationship between the height ofthe emitter tip and the thickness of the gate electrode, obtained by asimulation analysis;

FIG. 14 is a cross-sectional view illustrating one step of the secondembodiment of the method for fabricating a cathode device according tothe present invention;

FIG. 15 is a cross-sectional view illustrating the next step after thatin FIG. 14;

FIG. 16 is a cross-sectional view illustrating the next step after thatin FIG. 15;

FIG. 17 is a cross-sectional view illustrating the next step after thatin FIG. 16;

FIGS. 18A to 18E are cross-sectional views illustrating the thirdembodiment of the method for fabricating a cathode device according tothe present invention;

FIGS. 19A to 19C are cross-sectional views illustrating the fourthembodiment of the method for fabricating a cathode device according tothe present invention;

FIGS. 20A to 20E are cross-sectional views illustrating the fifthembodiment of the method for fabricating a cathode device according tothe present invention;

FIGS. 21A to 21C are cross-sectional views illustrating the sixthembodiment of the method for fabricating a cathode device according tothe present invention;

FIGS. 22A to 22D are cross-sectional views illustrating the seventhembodiment of the method for fabricating a cathode device according tothe present invention;

FIGS. 23A to 23C are cross-sectional views illustrating the eighthembodiment of the method for fabricating a cathode device according tothe present invention;

FIGS. 24A to 24C are cross-sectional views illustrating the ninthembodiment of the method for fabricating a cathode device according tothe present invention;

FIGS. 25A to 25C are cross-sectional views illustrating the tenthembodiment of the method for fabricating a cathode device according tothe present invention;

FIG. 26 is a cross-sectional view illustrating a modified example of theetching step of FIG. 6;

FIG. 27 is a cross-sectional view illustrating a modified example of thegate electrode forming step of FIG. 16;

FIG. 28 is a cross-sectional view of a cathode device of the prior art;

FIGS. 29A to 29D are cross-sectional views illustrating the method forfabricating a cathode device according to the prior art;

FIG. 30 is a cross-sectional view illustrating the method forfabricating a cathode device according to another prior art; and

FIG. 31 is a cross-sectional view illustrating the cathode device of theprior art fabricated by the step shown in FIG. 30.

THE DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a cathode device 100 according to the present invention.The cathode device 100 comprises a substrate 1, an emitter tip 2 formedon the substrate 1, an insulating layer 3, and a gate electrode layer 4.An oxide layer 52 is interposed between the substrate 1 and theinsulating layer 3. The emitter tip 2 comprises a conical tip endportion 2A and a cylindrical stem portion 2B. Alternatively, the emittertip 2 may be arranged to have the conical tip end portion 2A only,without the cylindrical stem portion 2B. The insulating layer 3 and thegate electrode layer 4 have openings 3A and 4B, respectively, whichalign with each other and through which the emitter tip 2 is exposed.The inner circumferential wall 4B of the opening 4A of the gateelectrode layer 4 surrounds the tip end portion 2A of the emitter tip 2.An electric field is induced between the tip end portion 2A of theemitter tip 2 and the inner circumferential wall 4B of the opening 4A ofthe gate electrode layer 4, whereby electrons are emitted from the tipend portion 2A of the emitter tip 2. Also, a cathode electrode (notshown) is connected to the stem portion 2B of the emitter tip 2.

In FIG. 1, the inner circumferential wall 4B of the opening 4A of thegate electrode 4 is formed on the outside the conical tip end portion 2Aof the emitter tip 2 in a substantially truncated cone and extendingsubstantially in parallel to the conical tip end portion 2A of theemitter tip 2. Therefore, the diameter of the inner circumferential wall4B of the opening 4A of the gate electrode layer 4 is smaller than thatof a portion of the emitter tip 2 at the juncture (bottom surface) ofthe emitter tip 2 with the substrate 1. The width of the innercircumferential wall 4B of the opening 4A of the gate electrode layer 4(viewed in the direction of the height of the emitter tip 2) is largerthan the thickness of the gate electrode layer 4 (on the insulatinglayer 3). A lowest point of the inner circumferential wall of theopening is below a surface of a portion of the gate electrode adjacentthe opening. The inner circumferential wall of the opening includes acircumferential lip therearound which has a cross section in a V-shape.An innermost surface of the circumferential lip forms a leg of theV-shape, and is parallel to a surface of the conical tip end portion ofthe emitter tip. One leg of the V-shape, including an upper edgethereof, is parallel to the emitter tip. Another leg of the V-shapeextends downwardly from the body of the gate electrode, and the one legof the V-shape extends upwardly.

FIG. 2 schematically shows an example in which the cathode device 100 ofFIG. 1 is used. In this example, the substrate 1 is made of, forexample, an n-type silicon semiconductor, and the n-type substrate 1 hasat the surface thereof regions la having an opposite conductivity, i.e.,a p-type conductivity. A plurality of emitter tips 2 are formed in eachp-type region 1a. In this case, the p-type regions 1a are used ascathode electrodes. When the voltage is applied between a certain p-typeregion la and the gate electrode layer 4, electrons are emitted from theemitter tips 2 in the p-type region 1a, as indicated by arrows. Theemitted electrons travel to an anode device 200 arranged in associationwith the cathode device 100.

FIG. 3 shows an example of a display device as an application of thecathode device 100 of FIGS. 1 and 2. P-type regions 1a and the gateelectrode layers 4 are arranged in a matrix with the insulating layer 3arranged therebetween. Electrons can be emitted from the emitter tips 2in a specific point in the matrix to the anode device 200. The anodedevice 200 comprises an anode layer 201 formed on a transparent plateand phosphor layers (R, G, and B) 202 formed on the anode layer 201.Electrons travelling from a specific point in the cathode device 100toward the anode layer 201 hit the phosphor layer 202 at a relatedpoint. A color display can thus be established. The cathode device 100is not limited to this application but may also be applied to variousdevices such as a micro-vacuum tube, as described previously.

FIGS. 4 to 10 show consecutive steps in the first embodiment of themethod for fabricating the cathode device 100 of FIG. 1.

In FIG. 4, a silicon substrate 1 is prepared. The silicon substrate 1 isof an n-type, and a p-type region (or regions) 1a is formed on thesurface of the silicon substrate 1, as described with reference to FIG.2. Alternatively, a cathode electrode made of another conductor may beformed on the silicon substrate 1 via an insulating layer.

A layer of the mask for forming the emitter tip 2 is formed on thesilicon substrate 1, for example, by a chemical vapor deposition (CVD)technique. In the embodiment, the mask layer comprises a two-layerstructure comprising, for example, a first insulating layer 61 having athickness of 300 nm and made of Si₃ N₄ and a second insulating layer 51having a thickness of 300 nm and made of SiO₂. The first insulatinglayer 61 is on the substrate 1, and the second insulating layer 51 is onthe first insulating layer 61. A nitride such as Si₃ N₄ is more easilydissolved in a thermal phosphoric acid than SiO₂. If the total thicknessis determined properly, the mask layer may be structured only with theSiO₂ insulating layer 51 or only with the Si₃ N₄ layer 61. The abovetwo-layer structure may be found advantageous especially when used inother embodiments, described later.

In FIG. 5, the first and second insulating layers 61 and 51 are etchedto obtain the predetermined mask pattern corresponding to the shape ofthe emitter tip 2 after they are exposed by a known exposure means. Theetching is performed using an RIE (reaction ion etching) method, and CF₄as the etching gas.

In FIG. 6, the silicon substrate 1 is etched to form the emitter tip 2,using the mask 61 and 51. In this case, SF₆ is used as the etching gasand an RIE method is adopted. After the etching, the unmasked portion ofthe surface of the substrate 1 is removed and the portion of thesubstrate 1 under the mask is left in the form of a peak which becomesthe the emitter tip 2.

In this embodiment, in order to form the emitter tip 2 having arelatively long shape comprising the conical tip end portion 2A andcylindrical stem portion 2B, etching close to isotropic etching isinitially performed to form the conical tip end portion 2A, andanisotropic etching is then performed to form the substantiallycylindrical stem portion 2B. Using the RIE method, the features ofisotropic etching and anisotropic etching can be controlled byappropriately selecting etching conditions such as pressure.

In FIG. 7, a thermal oxidation process is adopted for forming the SiO₂oxide layer 52 on the surface of the the silicon substrate 1. The oxidelayer 52 is formed by oxide diffusing into the silicon substrate 1 andthe emitter tip 2. The oxide diffuses into the surfaces (shown at thestep in FIG. 6) of the silicon substrate 1 and the emitter tip 2. Whenthe oxide layer 52 is removed subsequently, the emitter tip 2 becomessharper.

The oxide layer 52 swells to the outside of the surfaces (shown at thestep in FIG. 6) of the silicon substrate 1 and the emitter tip 2.Therefore, in the stage in which the mask 61 and 51 is not removed, theoutline of the oxide layer 52 on the cylindrical stem portion 2B of theemitter tip 2 is a slightly larger than the outline of the mask 61 and51 (line extended from the mask 61 and 51 is indicated by the dashedline in FIG. 7). That is, a shoulder of the oxide layer 52 above thecylindrical stem portion 2B of the emitter tip 2 projects outside theouter contour of the mask 61 and 51.

In FIG. 8, a 1000 nm thick insulating layer 3 of SiO₂ is formed using avacuum deposition method. The contour of the insulating layer 3 on themask 61 and 51 becomes divergent in the radial direction as it piles up,and the contour of the insulating layer 3 on the oxide layer 52 on thesubstrate 1 which is not covered by the mask 61 and 51 becomes divergentcorrespondingly.

During the formation of the insulating layer 3, a material of theinsulating layer 3 is deposited to the shoulder of the oxide layer 52 onthe cylindrical stem portion 2B of the emitter tip 2, with the resultthat a growing collar 10 is parasitically formed. The growing collar 10is formed because the oxide layer 52 on the cylindrical stem portion 2Bof the emitter tip 2 is protruding beyond the contour of the mask 61 and51. In some of the embodiments described later, the growing collar 10 isnot formed during the formation of the insulating layer 3.

In FIG. 9, the Cr gate electrode layer 4 having the thickness of 200 nmis added using a vacuum deposition method.

In this embodiment, the presence of the growing collar 10 is utilizedactively during the step of forming the gate electrode layer 4. That isto say, the growing collar 10 is used as a mask so that a gate electrodematerial will reach the vicinity of the oxide layer 52 on the tip endportion 2A of the emitter tip 2 above the upper surface of the growingcollar 10 in order that a great amount of the gate electrode material isdeposited on the oxide layer 52 on the conical tip end portion 2A of theemitter tip 2.

In the step of forming the gate electrode layer 4, oblique deposition ispreferably carried out. Specifically, while the substrate 1 is beingrotated about the center axis (rotation axis), the gate electrodematerial is deposited on the substrate 1 in the direction (arrow A)oblique to the rotation axis. Therefore, a great amount of the gateelectrode material reaches the oxide layer 52 on the tip end portion 2Aof the emitter tip 2, passing through the passage above the uppersurface of the growing collar 10. The gate electrode material is thendeposited on the oxide layer 52.

Sputtering or CVD methods can be adopted in the step of forming the gateelectrode layer 4, on behalf of the vacuum deposition method. When thesputtering or CVD method is employed, a great amount of the gateelectrode material easily reaches the vicinity of the oxide layer 52 onthe tip end portion 2A of the emitter tip 2, passing through the passageabove the upper surface of the growing collar 10, and is then depositedthe oxide layer 52. Furthermore, the treating conditions can be selectedso that a great amount of the gate electrode material surely reach thevicinity of the oxide layer 52. For example, as long as the sputteringmethod is utilized, the treating pressure can be raised to 20 mtorr,compared with the standard treating pressure of 5 mtorr, or the distancebetween the substrate 1 and the target is reduced to 70 mm, comparedwith the standard distance of 120 mm, so that the solid angle of thetarget with respect to the substrate 1 will be increased. If thethickness of the gate electrode layer is increased, the amount of thedeposited gate electrode material will be increased.

Thereafter, the mask 61 and 51, the oxide layer 52 on the surface of theconical tip end portion 2A of the emitter tip 2, and the growing collar10 are removed, as shown in FIG. 10. In this embodiment, the substrate 1is immersed in a hydrofluoric acid etchant to cause these layers to bedissolved. When the oxide layer 52 dissolves, the sharp emitter tip 2appears. The insulating layer 3 and the gate electrode layer 4eventually have openings 3A and 4A through which the emitter tip 2 isexposed.

The gate electrode layer 4 is then subjected to wet etching into anappropriate pattern (see FIG. 3). The cathode device 100 in FIG. 10 isidentical to that shown in FIG. 1. In FIG. 10, the diameter of the innercircumferential wall 4B of the opening 4A of the gate electrode layer 4is indicated by "d", and the diameter of the portion of the emitter tip2 at the juncture with the substrate 1 is indicated by "D". According tothe present invention, "d" is smaller than "D". In addition, the width"W" of the inner circumferential wall 4B of the opening 4A of the gateelectrode layer 4 is larger than the thickness "t" of the gate electrodelayer 4.

FIG. 11 shows the relationship between voltage and current, measured inthe cathode devices of the present invention and the prior art. Theabscissas indicates the voltage and the ordinate indicates the emissioncurrent. Circular marks indicate values concerning the presentinvention, and square marks indicate values concerning the prior art. Inthe measurement, 6,400 emitter tips were set in array and operated, andthe result of measurements is shown. As can seen in FIG. 11, the cathodedevice relating to the present invention starts emitting, abruptly, at alower voltage than the the cathode device of the prior art.

FIG. 12 graphically shows field emission characteristics based on theFowler-Nordheim's rule. The abscissas indicates 1000/V, and the ordinateindicates I/V². When a logarithmic function between both sides of theFowler-Nordheim's formula presented below represents a linear line, thecharacteristic is regarded as a field emission characteristic. ##EQU1##where, Je is a current density, φ is a work function, A and B areconstants, and E is an electric Field.

For plotting the graph of FIG. 12, 6,400 emitter tips were set in arrayand operated, and the result of the measurements is shown. Circularmarks indicate values concerning the present invention, and square marksindicate values concerning the prior art.

It is known that a satisfactory result is obtained if the characteristicline has a small gradient and high values. As seen in FIG. 12, thecathode device relating to the present invention yields a larger currentwith a smaller voltage than the cathode device of the prior art.

FIG. 13 shows the relationship between the height of the emitter tip 2and the field enhancement factor, obtained by simulation analysis.Curves P, Q, and R represent field enhancement factors under conditionswhere the width W of the inner circumferential wall 4B of the opening 4Ain the gate electrode layer 4 is set to 0.2, 0.4 or 0.6 um, wherein thesum of thicknesses of the oxide layer 52 and insulating layer 3 in FIG.1 is 1 um. The curves P, Q, and R have peak values when the height ofthe emitter tip 2 is about 1.41 um. Among the field enhancement factors,the field enhancement factor related to a larger width W of the innercircumferential wall 4B has a larger value (curve R>curve Q>curve P).Thus, according to the present invention, since the innercircumferential wall 4B of the opening 4A in the gate electrode layer 4is formed in the form of a substantially truncated cone extendingsubstantially in parallel to the conical tip end portion 2A of theemitter tip 2, the width W of the inner circumferential wall 4B is largeand therefore the field multiplication is large.

FIGS. 14 to 17 show consecutive steps in the second embodiment of themethod for fabricating the cathode device 100. The steps shown in FIGS.4 to 7 in the first embodiment are similarly adopted in the secondembodiment. FIG. 14 shows the same condition as that of FIG. 8 in thefirst embodiment. That is, FIG. 14 shows the condition after the mask 61and 51 for the emitter tip 2 is formed on the silicon substrate 1 andgiven a desired mask pattern; the substrate 1 is etched using the mask61 and 51 to form the emitter tip 2; the oxide layer 52 is formed on thesurfaces of the silicon substrate 1 and the emitter tip 2; and theinsulating layer 3 is formed. When the insulating layer 3 is formed, thegrowing collar 10 is parasitically formed with the insulating layer 3.

In this embodiment, as shown in FIG. 15, the substrate 1 is immersed inan etchant of thermal phosphoric acid after the insulating layer 3 isformed. Since a nitride such as Si₃ N₄ is more soluble in thermalphosphoric acid than SiO₂, the first insulating layer 61 in the mask 61is easily dissolved and removed from the substrate 1, and secondinsulating layer 51 of the mask is removed together with the mask 61. Onthe contrary, the insulating layer 3 and growing collar 10 are notdissolved but left unremoved.

In FIG. 16, the Cr gate electrode layer 4 having a thickness of 200 nm,is formed using a vacuum deposition method. In this embodiment, sincethe mask 61 and 51 were removed, a material of the gate electrode layer4 is deposited to the growing collar 10 and the oxide layer 52 on thetip end portion 2A of the emitter tip 2, and gradually grows.

In FIG. 17, the substrate 1 is immersed in a hydrofluoric acid etchantin order to remove the oxide layer 52 covering the emitter tip 2 and thegrowing collar 10. The etchant penetrates into a gap in the gateelectrode material at the upper edge of the oxide layer 54 on the tipend portion 2A of the emitter tip 2 or a portion of the gate electrodematerial that is so thin as to create a gap. Thus, the oxide layer 52 isremoved. In a modified example, if the tip end portion 2A of the emittertip 2 is formed under an appropriately selected RIE condition in theemitter tip forming step so that the tip end portion 2A has a narrowpart 2C, as shown in FIG. 26 which show the step corresponding to thestep of FIG. 6, a large gap 4G is created in the gate electrode layer 4,as shown in FIG. 27 which show the step corresponding to the step ofFIG. 16. Accordingly, the removal of the oxide layer 52 performed in thestep of FIG. 17 is facilitated.

Once the oxide layer 52 and the growing collar 10 have been removed inthis way, the openings 4A and 3A are created in the gate electrode layer4 and the insulating layer 3 around the tip end portion 2A of theemitter tip 2, respectively. The diameter of the opening 4A of the gateelectrode layer 4 formed according to the second embodiment is smallerthan that created according to the first embodiment.

FIGS. 18A to 18E show the third embodiment of the method for fabricatingthe cathode device 100. FIG. 18A shows a condition after the mask 61 and51 for the emitter tip 2 is formed on the silicon substrate 1 and givena desired mask pattern; the substrate 1 is etched using the mask 61 and51 to form the emitter tip 2; and the insulating layer 3 is formed. Inthis embodiment, the insulating layer 3 is formed prior to the formationof the oxide layer 52. The emitter tip 2 is relatively short andsubstantially consists of the conical tip end portion 2A alone. In thiscase, since the shoulder of the stem portion 2B described in the firstembodiment does not exist, the growing collar 10 is not parasiticallyformed.

In FIG. 18B, the substrate 1 is immersed in a thermal phosphoric acidetchant after the insulating layer 3 is formed. In this case too, anitride such as Si₃ N4 is more soluble in thermal phosphoric acid thanSiO₂. The mask 61 and 51 is therefore removed from the substrate 1,while the insulating layer 3 is not dissolved but left unremoved.

In FIG. 18C, the oxide layer 52 is formed on the surfaces of the siliconsubstrate 1 and the emitter tip 2 to sharpen the emitter tip 2.

In FIG. 18D, the gate electrode layer 4 is formed. In this embodiment,since the mask 61 and 51 has already been removed and the growing collar10 does not exist, the gate electrode layer 4 is fully deposited on theoxide layer 52 on the tip end portion 2A of the emitter tip 2.

In FIG. 18E, the oxide layer 52 covering the emitter tip 2 is removedusing the etchant. The etchant penetrates into the gap created in thegate electrode layer 4 at the upper edge of the oxide layer 52 on thetip end portion 2A of the emitter tip 2 or a portion of the gateelectrode layer 4 that is so thin as to create a gap, whereby the oxidelayer 52 is removed. In this case too, if the tip end portion 2A of theemitter tip 2 is formed so that the tip end portion 2A has a narrow part2C, as shown in FIG. 26, a large gap 4G is created in the gate electrodelayer 4, as shown in FIG. 27, to facilitate the removal of the oxidelayer 52. In this case too, the diameter of the opening 4A in the gateelectrode layer is smaller than that of the bottom of the emitter tip 2.

FIGS. 19A to 19C show the fourth embodiment of the method forfabricating the cathode device 100. This embodiment is substantiallyidentical to the embodiment shown in FIGS. 18A to 18E. FIG. 19A shows acondition after the mask 61 and 51 for the emitter tip 2 is formed onthe silicon substrate 1 and given a desired mask pattern; the substrate1 is etched using the mask 61 and 51 to form the emitter tip 2; and theinsulating layer 3 is formed. Here, the emitter tip 2 comprises theconical tip end portion 2A and the cylindrical stem portion 2B. Theoxide layer 52 has not yet been formed in this stage. Since the oxidelayer 52 has not yet been formed, the emitter tip 2 does not swell (theemitter tip 2 may swell due to diffusion during the oxide layer formingstep) and the shoulder of the emitter tip does not protrude beyond thecontour of the mask 61 and 51. Therefore, the growing collar 10 is notparasitically produced in the insulating layer forming step.

In FIG. 19B, the substrate 1 is then immersed in a thermal phosphoricacid etchant to remove the mask 61 and 51 from the substrate 1. Theinsulating layer 3 is not dissolved but left unremoved. Thereafter, theoxide layer 52 is formed on the surfaces of the silicon substrate 1 andthe emitter tip 2, which is intended to sharpen the emitter tip 2.

The gate electrode layer 4 is then formed. In this embodiment, since themask 61 and 51 have been already removed and the growing collar 10 doesnot exist, the gate electrode layer 4 is fully deposited on the oxidelayer 52 on the tip end portion 2A of the emitter tip 2.

In FIG. 19C, the oxide layer 52 covering the emitter tip 2 is removed,using the etchant. Even in this embodiment, the diameter of the openingA of the gate electrode layer 4 is smaller than that of the bottom ofthe emitter tip 2.

FIGS. 20A to 20E show the fifth embodiment of the method for fabricatingthe cathode device 100. FIG. 20A shows a condition after the mask 61 and51 for the emitter tip 2 is formed on the silicon substrate 1 and givena desired mask pattern; the substrate 1 is etched using the mask 61 and51 to form the emitter tip 2; and the oxide layer 52 is formed on thesurfaces of the silicon substrate 1 and the emitter tip 2 to sharpen theemitter tip 2. In this embodiment, the emitter tip 2 is relatively shortand substantially consists of the conical tip end portion 2A alone.

In FIG. 20B, the insulating layer 3 is formed after the oxide layer 52has been formed. In this embodiment, since the shoulder of the emittertip does not exist, the growing collar 10 is not parasitically producedin the insulating layer forming step.

In FIG. 20C, the substrate 1 is immersed in a thermal phosphoric acidetchant. The mask 61 and 51 is thus removed, while the insulating layer3 is not dissolved but left unremoved.

In FIG. 20D, the gate insulating layer 4 is formed. Even in thisembodiment, since the mask 61 and 51 have been removed and the growingcollar 10 does not exist, the gate electrode layer 4 is fully depositedon the oxide layer 52 on the tip end portion 2A of the emitter tip 2.

In FIG. 20E, the oxide layer 52 covering the emitter tip 2 is removedusing the etchant. The etchant penetrates into a gap created in the gateelectrode material at the upper edge of the oxide layer 52 on the tipend portion 2A of the emitter tip 2 or a portion of the gate electrodematerial that is so thin as to create a gap, whereby the oxide layer 52is removed. In this case too, if the tip end portion 2A of the emittertip 2 is formed so that the tip end portion 2A has a narrow part 2C, asshown in FIG. 26, a large gap 4G is created in the gate electrode layer4, as shown in FIG. 27, to facilitate the removal of the oxide layer 52.Even in this embodiment, the diameter of an opening 4A in the gateelectrode layer is smaller than that of the bottom of the emitter tip 2.

FIGS. 21A to 21C show the sixth embodiment of the method for fabricatingthe cathode device 100. This embodiment is substantially identical tothat in FIG. 20A to 20E. FIG. 21A shows a condition after the mask 61and 51 for the emitter tip 2 is formed on the silicon substrate 1 andgiven a desired mask pattern; the substrate 1 is etched using the mask61 and 51 to form the emitter tip 2; the oxide layer 52 is formed on thesurfaces of the silicon substrate 1 and the emitter tip 2 to sharpen theemitter tip 2; and the insulating layer 3 is formed. In this embodiment,the emitter tip 2 comprises the conical tip end portion 2A and the.cylindrical stem portion 2B, and the growing collar 10 is parasiticallyformed.

In FIG. 21B, the substrate 1 is immersed in an etchant of thermalphosphoric acid to remove the mask 61 and 51. The etching is such thatthe insulating layer 3 is not dissolved but the growing collar 10 isdissolved because the etching rate of the SiO₂ layer becomes higher byraising the temperature of the thermal phosphoric acid.

Thereafter, the gate electrode layer 4 is formed. In this embodiment,since the mask 61 and 51 has already been removed and the growing collar10 is not created, the gate electrode layer 4 is fully deposited to theoxide layer 52 on the tip end portion 2A of the emitter tip 2.

In FIG. 21C, the oxide layer 52 covering the emitter tip 2 is removedusing the etchant. Even in this embodiment, the diameter of the opening4A of the gate electrode layer 4 is smaller than that of the bottom ofthe emitter tip 2.

FIGS. 22A to 22D show the seventh embodiment of the method forfabricating the cathode device 100. FIG. 22A shows a condition after themask 61 and 51 for the emitter tip 2 is formed on the silicon substrate1 and given a desired mask pattern; and the substrate 1 is etched usingthe mask 61 and 51 to form the emitter tip 2.

In FIG. 22B, the substrate 1 is immersed in an etchant of thermalphosphoric acid or hydrofluoric acid to remove the mask 61 and 51. Theoxide layer 52 is then formed on the surfaces of the silicon substrate 1and the emitter tip 2 to sharpen the emitter tip 2. The emitter tip 2substantially comprises the conical tip end portion 2A alone. In thiscase, the mask may comprise only one insulating layer 51 made of SiO₂.

In FIG. 22C, the gate electrode layer 4 is formed after the oxide layer52 has been formed. Even in this embodiment, since the mask 61 and 51has been removed and the growing collar 10 is not created, the gateelectrode layer 4 is fully deposited onto the oxide layer 52 on the tipend portion 2A of the emitter tip 2.

In FIG. 22D, the oxide layer 52 covering the emitter tip 2 is removedusing the etchant. The etchant penetrates into a gap created in the gateelectrode material at the upper edge of the oxide layer 52 on the tipend portion 2A of the emitter tip 2 or a portion of the gate electrodematerial that is so thin as to create a gap, whereby the oxide layer 52is removed. In this case too, if the tip end portion 2A of the emittertip 2 is formed so that the tip end portion 2A has a narrow part 2C, asshown in FIG. 26, a large gap 4G is created in the gate electrode layer4, as shown in FIG. 27, to facilitate the removal of the oxide layer 52.However, a portion of the oxide layer 52 on the surface of the substrate1 except for the position on which the emitter tip is located is notdissolved but left unremoved. The unremoved oxide layer 52 serves as aninsulating layer for isolating the substrate 1 from the gate electrodelayer 4. Even in this embodiment, the diameter of the opening 4A of thegate electrode layer is smaller than that of the bottom of the emittertip 2.

FIGS. 23A to 23C show the eighth embodiment of the method forfabricating the cathode device 100. This embodiment is substantiallyidentical to that in FIGS. 22A to 22D. FIG. 23A shows a condition afterthe mask 61 and 51 for the emitter tip 2 is formed on the siliconsubstrate 1 and given a desired mask pattern; the substrate 1 is etchedusing the mask 61 and 51 to form the emitter tip 2; and the oxide layer52 is formed on the surfaces of the silicon substrate 1 and the emittertip 2 to sharpen the emitter tip 2 after the mask 61 and 51 is removed.

In FIG. 23B, the insulating layer 3 is formed after the oxide layer 52if formed, and thereafter the gate electrode layer 4 is formed.

In FIG. 23C, the oxide layer 52 and the insulating layer 3 covering theemitter tip 2 are removed using an etchant. Even in this embodiment, thediameter of an opening 4A in the gate electrode layer is smaller thanthat of the bottom of the emitter tip 2.

FIGS. 24A to 24C show the ninth embodiment of the method for fabricatingthe cathode device 100. This embodiment is substantially identical tothat shown in FIGS. 22A to 22D, except that the emitter tip 2 comprisesthe conical tip end portion 2A and the cylindrical stem portion 2B. InFIG. 24A, the substrate 1 is etched using the mask 61 and 51 to form theemitter tip 2, and the oxide layer 52 is formed on the surfaces of thesilicon substrate 1 and the emitter tip 2 after the mask 61 and 51 isremoved.

In FIG. 24B, the gate electrode layer 4 is formed after the oxide layer52 is formed.

In FIG. 24C, the oxide layer 52 covering the emitter tip 2 is removedusing an etchant. However, a portion of the oxide layer 52 on thesurface of the substrate 1 except for the position on which the emittertip 2 is located is not dissolved but left unremoved, and the unremovedportion of the oxide layer 52 serves as an insulating layer forisolating the substrate 1 from the gate electrode layer 4. Even in thisembodiment, the diameter of the opening 4A of the gate electrode layeris smaller than that of the bottom of the emitter tip 2.

FIG. 25A to 25C show the tenth embodiment of the method for fabricatingthe cathode device 100. This embodiment is substantially identical tothat shown in FIGS. 23A to 23C except that the emitter tip 2 comprisesthe conical tip end portion 2A and the cylindrical stem portion 2B. InFIG. 24A, the substrate 1 is etched using the mask 61 and 51 to form theemitter tip 2, and the oxide layer 52 is formed on the surfaces of thesilicon substrate 1 and the emitter tip 2 after the mask 61 and 51 isremoved.

In FIG. 25B, the insulating layer 3 is formed after the oxide layer 52if formed, and thereafter the gate electrode layer 4 is formed.

In FIG. 25C, a portion of the oxide layer 52 and the insulating layer 3covering the emitter tip 2 are removed using an etchant. Even in thisembodiment, the diameter of an opening 4A in the gate electrode layer issmaller than that of the bottom of the emitter tip 2.

As explained in greater detail, according to the present invention, thediameter of the inner circumference of the opening in the gate electrodelayer is smaller than that of a portion of the emitter tip at thejuncture with the substrate. Accordingly, it is possible to producecathode device capable of inducing field emission with a low appliedvoltage. Using this cathode device, a high definition display device canbe constructed. Moreover, since the gate electrode layer can be formedclose to the tip end portion of the emitter tip, the effect ofconcentration of the electric field is markedly improved and the voltagenecessary for actuating the small-sized field emission cold cathode canbe lowered.

We claim:
 1. A method for fabricating a cathode device, the method comprising the steps of:forming at least one emitter tip having a conical tip end portion on a substrate, said step of forming comprising the steps of forming a mask on the substrate, the mask having a shape corresponding to the shape of the emitter tip, performing etching on the substrate using the mask, and then removing the mask, wherein at least part of the mask is made of a different material than the oxide layer, and wherein, during the step of removing the mask, the mask is selectively dissolved relative to the oxide layer; said method further comprising the steps offorming an oxide layer at least on the surface of the formed emitter tip; forming a gate electrode layer such that a material for the gate electrode is deposited at least to the oxide layer on the surface of the conical tip end portion of the emitter tip and the gate electrode layer has an opening through which the tip end portion of the emitter tip is exposed; and removing the oxide layer on the surface of the emitter tip so that an inner circumferential wall of the opening of the gate electrode layer is formed on the outside of the conical tip end portion of the emitter tip and extends approximately in parallel to the conical tip end portion of the emitter tip, wherein the step of removing the mask is performed prior to the step of forming the gate electrode layer.
 2. A method for fabricating a cathode device according to claim 1, wherein in the step of forming the gate electrode layer, the inner circumferential wall of the opening of the gate electrode layer is formed in a substantially truncated cone on the outside of the conical tip end portion of the emitter tip and extends substantially in parallel to the conical tip end portion of the emitter tip.
 3. A method for fabricating a cathode device according to claim 1, wherein in the step of forming the gate electrode layer, the material for the gate electrode layer is vapor deposited onto the substrate, while the substrate is rotated about a rotation axis, in the direction oblique to the rotation axis.
 4. A method for fabricating a cathode device according to claim 1, wherein in the step of forming the gate electrode layer, the material for the gate electrode layer is deposited to the substrate by sputtering.
 5. A method for fabricating a cathode device according to claim 1, wherein in the step of forming the gate electrode layer, the material for the gate electrode layer is deposited to the substrate by CVD method.
 6. A method for fabricating a cathode device according to claim 1, wherein in the step of forming the emitter tip, an isotropic etching is initially performed and an anisotropic etching is subsequently performed, whereby an emitter tip having a conical tip end portion and a substantially cylindrical stem portion is formed.
 7. A method for fabricating a cathode device according to claim 1, further comprising the step of forming an insulating layer between the substrate and the gate electrode layer, the insulating layer having an opening through which the emitter tip is exposed.
 8. A method for fabricating a cathode device according to claim 7, wherein the step of forming the insulating layer includes the step of forming a growing collar that parasitically grows from an intermediate portion of the emitter tip, and the growing collar is used as a mask in the step of forming the gate electrode layer so that the material of the gate electrode reaches the vicinity of the tip end portion of the emitter tip above the growing collar.
 9. A method for fabricating a cathode device according to claim 1, wherein the step of forming the oxide layer comprises a thermal oxidation process.
 10. A method for fabricating a cathode device according to claim 9, wherein an oxide layer is formed on the surfaces of the emitter tip and the substrate in the step of forming the oxide layer, the oxide layer on the surface of the substrate acting as an insulating layer between the substrate and the gate electrode layer.
 11. A method for fabricating a cathode device according to claim 9, wherein the method comprises the step of forming an insulating layer between the substrate and the gate electrode layer, differently from the oxide layer on the surface of the substrate, the step of forming the insulating layer being performed after the step of forming the oxide layer.
 12. A method for fabricating a cathode device according to claim 9, wherein the method comprises the step of forming an insulating layer between the substrate and the gate electrode layer, different from the oxide layer on the surface of the substrate, the step of forming the insulating layer being performed prior to the step of forming the oxide layer.
 13. A method for fabricating a cathode device, the method comprising the steps of:forming at least one emitter tip having a conical tip end portion on a substrate; forming an oxide layer at least one the surface of the formed emitter tip; forming a gate electrode layer such that a material for the gate electrode is deposited at least to the oxide layer on the surface of the conical tip end portion of the emitter tip and the gate electrode layer has an opening through which the tip end portion of the emitter tip is exposed; removing the oxide layer on the surface of the emitter tip so that an inner circumferential wall of the opening of the gate electrode layer is formed on the outside of the conical tip end portion of the emitter tip and extends approximately in parallel to the conical tip end portion of the emitter tip; and forming an insulating layer between the substrate and the gate electrode layer, the insulating layer having an opening through which the emitter tip is exposed.
 14. A method for fabricating a cathode device according to claim 13, wherein the step of forming the insulating layer includes the step of forming a growing collar that parasitically grows from an intermediate portion of the emitter tip, and the growing collar is used as a mask in the step of forming the gate electrode layer so that the material of the gate electrode reaches the vicinity of the tip end portion of the emitter tip above the growing collar.
 15. A method for fabricating a cathode device according to claim 13, wherein in the step of forming the gate electrode layer, the inner circumferential wall of the opening of the gate electrode layer is formed in a substantially truncated cone on the outside of the conical tip end portion of the emitter tip and extends substantially in parallel to the conical tip end portion of the emitter tip.
 16. A method for fabricating a cathode device according to claim 13, wherein the step of forming the emitter tip comprises the steps of forming a mask on the substrate, the mask having a shape corresponding to the shape of the emitter tip, performing etching on the substrate using the mask, and removing the mask;the method comprising the step of forming an insulating layer between the substrate and the gate electrode layer prior to the step of forming the oxide layer, the insulating layer having an opening through which the tip end portion of the emitter tip is exposed; and the step of removing the mask being performed after the step of forming the insulating layer and prior to the step of forming the oxide layer, and the step of forming the gate electrode layer being performed after the step of removing the mask.
 17. A method for fabricating a cathode device according to claim 13, wherein the step of forming the emitter tip comprises the steps of forming a mask on the substrate, the mask having a shape corresponding to the shape of the emitter tip, performing etching on the substrate using the mask, and removing the mask;the method comprising the step of forming an insulating layer between the substrate and the gate electrode layer after the step of forming the oxide layer, the insulating layer having an opening through which the tip end portion of the emitter tip is exposed; and the step of removing the mask being performed prior to the step of forming the oxide layer, so that the oxide layer on the substrate becomes an insulating layer between the substrate and the gate electrode layer.
 18. A method for fabricating a cathode device according to claim 13, wherein the step of forming the emitter tip comprises the steps of forming a mask on the substrate, the mask having a shape corresponding to the shape of the emitter tip, performing etching on the substrate using the mask, and removing the mask;the method comprising the step of forming an insulating layer between the substrate and the gate electrode layer prior to the step of forming the oxide layer, the insulating layer having an opening through which the tip end portion of the emitter tip is exposed; and the step of removing the mask being performed prior to the step of forming the oxide layer.
 19. A method for fabricating a cathode device according to claim 13, wherein in the step of forming the gate electrode layer, the material for the gate electrode layer is vapor deposited onto the substrate, while the substrate is rotated about a rotation axis, in the direction oblique to the rotation axis.
 20. A method for fabricating a cathode device according to claim 13, wherein in the step of forming the gate electrode layer, the material for the gate electrode layer is deposited to the substrate by sputtering.
 21. A method for fabricating a cathode device according to claim 13, wherein in the step of forming the gate electrode layer, the material for the gate electrode layer is deposited to the substrate by CVD method.
 22. A method for fabricating a cathode device according to claim 13, wherein in the step of forming the emitter tip, an isotropic etching is initially performed and an anisotropic etching is subsequently performed, whereby an emitter tip having a conical tip end portion and a substantially cylindrical stem portion is formed.
 23. A method for fabricating a cathode device according to claim 13, wherein the step of forming the emitter tip comprises the steps of forming a mask on the substrate, the mask having a shape corresponding to the shape of the emitter tip, performing etching on the substrate using the mask, and removing the mask;the method comprising the step of forming an insulating layer between the substrate and the gate electrode layer being performed after the step of forming the oxide layer, the insulating layer having an opening through which the tip end portion of the emitter tip is exposed; and the step of removing the mask and the step of removing the oxide layer being performed after the step of forming the gate electrode layer.
 24. A method for fabricating a cathode device according to claim 23, wherein the step of forming the insulating layer includes the step of forming a growing collar that parasitically grows from an intermediate portion of the emitter tip, the growing collar being used as a mask in the step of forming the gate electrode layer so that a material of the gate electrode reaches the vicinity of the tip end portion of the emitter tip above the growing collar, the growing collar with the mask and the oxide layer around the emitter tip being removed after the step of forming the gate electrode layer.
 25. A method for fabricating a cathode device accoridng to claim 13, wherein the step of forming the emitter tip comprises the steps of forming a mask on the substrate, the mask having a shape corresponding to the shape of the emitter tip, performing etching on the substrate using the mask, and removing the mask;the method comprising the step of forming an insulating layer between the substrate and the gate electrode layer being performed after the step of forming the oxide layer and prior to the step of forming the gate electrode layer, the insulating layer having an opening through which the tip end portion of the emitter tip is exposed; and the step of removing the mask being performed after the step of forming the insulating layer and prior to the step of forming the gate electrode layer, and the step of removing the oxide layer being performed after the step of forming the gate electrode layer.
 26. A method for fabricating a cathode device according to claim 25, wherein the step of forming the insulating layer includes the step of forming a growing collar that parasitically grows from an intermediate portion of the emitter tip, and the growing collar is used as a mask in the step of forming the gate electrode layer so that a material of the gate electrode reaches the vicinity of the tip end portion of the emitter tip above the growing collar, the growing collar with the mask and the oxide layer around the emitter tip being removed after the step of forming the gate electrode layer.
 27. A method for fabricating a cathode device according to claim 13, wherein the step of forming the emitter tip comprises the steps of forming a mask on the substrate, the mask having a shape corresponding to the shape of the emitter tip, performing etching on the substrate using the mask, and removing the mask;the method comprising the step of forming an insulating layer between the substrate and the gate electrode layer after the step of forming the oxide layer, the insulating layer having an opening through which the tip end portion of the emitter tip is exposed; and the step of removing the mask being performed after the step of forming the insulating layer.
 28. A method for fabricating a cathode device according to claim 27, wherein the step of forming the insulating layer includes the step of forming a collar that parasitically grows from an intermediate portion of the emitter tip, and the growing collar is used as a mask in the step of forming the gate electrode layer so that the material for the gate electrode reach the vicinity of the tip end portion of the emitter tip above the collar, the collar with the mask and the oxide layer around the emitter tip being removed after the step of forming the gate electrode layer.
 29. A method for fabricating a cathode device according to claim 13, wherein the step of forming the oxide layer comprises a thermal oxidation process.
 30. A method for fabricating a cathode device according to claim 29, wherein an oxide layer is formed on the surfaces of the emitter tip and the substrate in the step of forming the oxide layer, the oxide layer on the surface of the substrate acting as an insulating layer between the substrate and the gate electrode layer.
 31. A method for fabricating a cathode device according to claim 29, wherein the method comprises the step of forming an insulating layer between the substrate and the gate electrode layer, differently from the oxide layer on the surface of the substrate, the step of forming the insulating layer being performed after the step of forming the oxide layer.
 32. A method for fabricating a cathode device according to claim 29, wherein the method comprises the step of forming an insulating layer between the substrate and the gate electrode layer, different from the oxide layer on the surface of the substrate, the step of forming the insulating layer being performed prior to the step of forming the oxide layer.
 33. A method for fabricating a cathode device according to claim 13, wherein the step of forming the emitter tip comprises the steps of forming a mask on the substrate, the mask having a shape corresponding to the shape of the emitter tip, performing etching on the substrate using the mask, and removing the mask.
 34. A method for fabricating a cathode device according to claim 33, wherein at least a part of the mask is made of a different material from the oxide layer, and in the step of removing the mask, the mask is selectively dissolved relative to the oxide layer.
 35. A method for fabricating a cathode device according to claim 34, wherein the step of removing the mask is performed after the step of forming the gate electrode layer.
 36. A method for fabricating a cathode device according to claim 34, wherein the step of removing the mask is performed prior to the step of forming the gate electrode layer. 